Treatment of sleep disordered breathing (SDB), such as obstructive sleep apnea (OSA), by a respiratory treatment apparatus such as a continuous positive airway pressure (CPAP) flow generator system involves a delivery of air (or other breathable gas) at pressures above atmospheric pressure to the airways of a human or other mammalian patient via a conduit and/or a mask. Typically, the mask fits over the mouth and/or nose of the patient, or it may be an under-nose style such as a nasal pillows or nasal cushion style mask. Pressurized air flows to the mask and to the airways of the patient via the nose and/or mouth. As the patient exhales, carbon dioxide gas may collect in the mask and breathing circuit. A washout vent in the mask or conduit may be implemented to refresh the gas in the circuit by virtue of the positive pressure maintained within the circuit. By providing adequate renewal of gas at the patient-end of the circuit, the patient's exhaled gas can be expelled from the mask to atmosphere.
The washout vent is normally located in the mask or substantially near the mask in a gas delivery conduit coupled to the mask. The washout of gas through the vent to the atmosphere removes exhaled gases to prevent carbon dioxide build-up. “Rebreathing” of exhaled carbon dioxide may be a health risk to the mask wearer. Rebreathing may occur of the contents of any circuit volume on the patient side of the vent (the circuit “deadspace”). This is most problematic for those patients whose tidal volume is not substantially larger than this “deadspace”. Rebreathing may also occur of any exhaled volume that extends beyond the vent back up the circuit away from the patient. Any of this exhaled gas that remains at the start of the next inspiration will represent a proportion of rebreathing. Whether such a residual exhaled volume exists or not depends on the degree of venting, the patient's tidal volume, and the breath pattern. Breathing patterns more likely to foster rebreathing are those with substantial tidal volumes and minimal end-expiratory pause (e.g., obstructive lung mechanics such as in COPD). Adequate gas washout may be achieved by selecting a vent size and configuration that allows a minimum safe washout flow at a low operating CPAP pressure, which typically can be as low as 4 cm H2O for adults and 2 cm H2O for children.
WO 2006/102708 describes an air delivery system with a vent valve that is controlled to maintain a substantially constant air flow in the air delivery conduit and the air flow generator.
WO2005/051468 describes a vent assembly for use with a mask assembly. The vent assembly includes a first vent, a second vent and a selector to switch the flow of exhaled gas from a patient between the first and second vents.
There is a need for a gas washout vent arrangement which allows for adequate venting of carbon dioxide while permitting efficient air delivery to the patient.